In the production of compressed air by drawing air from the atmosphere and compressing it, copressed air which is supersaturated in water vapor is obtained due to known physical relationships, thus leading to the formation of water of condensation which must be removed from the system. After the removal of the water of condensation, however, there still remains compressed air which is saturated with water vapor so that more water of condensation is again produced upon a slight decrease in the temperature. Furthermore in the ordinary plants for the production of compressed air such as is used in many companies it can generally not be avoided that some of the grease and oil used for lubricating the compressor will enter the compressed air or that the compressed air furthermore contains particles of dust and microorganisms which are drawn in by the compressor together with the atmospheric air. In many cases it is permissible for the compressed air to be relatively moist and dirty. This is true in particular in cases in which relatively robust compressed air units are to be driven or where the compressed air is used for the cleaning of machine parts, as for instance in automobile workshops. On the other hand, there are a number of uses in which compressed air which is not only dry but also free of dust and microorganisms is required. This is true in particular for the use of compressed air in the field of medicine, for instance for the driving of drills for dentists. The situation is similar in the case of sensitive pneumatic controls in which the formation of water of condensation and the occurrence of particles of dust would cause disturbances in operation. The presence of traces of oils and greases in the compressed air is also frequently not permissible since such residues may lead to disturbances in operation or, as for instance in dentistry, can lead to a taste or odor which is annoying to the patient.
In view of the frequent demand for dry compressed air which is very clean and--particularly in the field of medicine--also free of microorganisms, plants for the production of dry air have been developed in which very favorable results are obtained with respect to the quality of the dry compressed air by the combining of at least one adsorption filter with additional filter devices for the filtering out of solids and oil residues.
One such plant is described, for instance, in U.S. Pat. No. 3,399,510 where the compressed air supplied by a compressor after it had passed through an oil and water separator is fed to an adsorption filter in which further mechanical filtration as well as very intensive drying are effected. In the known dry air plant a compressed air storage tank as well as an auxiliary tank are fed from the output of the adsorption filter. The auxiliary tank serves to receive a predetermined quantity of compressed air which is returned into the atmosphere through the adsorption filter and a tap opening provided in the region of the oil and water separator immediately upon the stopping of the compressor as the result of a predetermined maximum pressure having been reached in the compressed air storage tank. The adsorption filter is regenerated by the dried air flowing from the auxiliary tank normally the same operating conditions are essentially present as were present at the start of the preceding operation cylce of the compressor. The known plant, however, fails when large quantities of compressed air are removed for a longer period of time from the compressed air tank since in this case the compressor operates for a very long time without a regeneration of the adsorption filter taking place. Furthermore the known plant is very unfavorable inasmuch as it is not capable of taking into account varying environmental conditions such as variations in the temperature and the humidity of the air, so that the amount of air in the auxiliary tank is too large for most cases but too small for some cases to bring about a sufficient regeneration of the adsorption filter, which on the one hand leads to an unfavorable efficiency and on the other hand to variations of the quality of the compressed air in the compressed air tank. Similar problems arise also in those plants which operate with two adsorption filters which are generally used alternately by means of a time control for filtration and drying and regenerating. Such a plant which has two adsorption filters is described, for instance, in German Pat. No. 1,282,608.